There is a rapidly growing market for high molecular weight polyester resin. For instance, the market for polyethylene terephthalate bottle resin is rapidly expanding. There is also a large demand for polyethylene terephthalate having a high molecular weight for use in making reinforcements for rubber articles, such as tire cords. High molecular weight polyethylene terephthalate resin can also be used in manufacturing trays for frozen food which can be heated in either microwave ovens or convection ovens. There is a demand for higher and higher molecular weight resin in this application. For instance, there is a demand for polyethylene terephthalate tray resin which has an intrinsic viscosity of greater than 1.0 dl/g. However, polyethylene terephthalate resins having intrinsic viscosities of greater than 1.0 are not widely available due to technical difficulties associated with producing such resins utilizing standard commercial polymerization techniques. These difficulties consequently result in such resins which are produced utilizing standard commercial techniques being very expensive.
High molecular weight polyesters are commonly produced from low molecular weight polyesters of the same composition by solid state polymerization. The low molecular weight polyesters which are used in such solid state polymerizations can be prepared by conventional melt polymerizations. Solid state polymerization is generally considered advantageous in that the handling of high molecular weight ultra-high viscosity molten polymers is eliminated during the solid state polymerization phase. Thermal degradation during the solid state portion of the polymerization is also essentially avoided.
In melt polymerizations, molecular weight increases by esterification as well as transesterification. During the initial polymerization, esterification predominates. As the molecular weight increases, most of the carboxyl ends are consumed by esterification and water is eliminated. The final polycondensation occurs by transesterification and the elimination of glycol. The attainment of high molecular weight is hindered by reduced diffusion of the glycol polymerization by-product due to increased polymer viscosity.
In the solid state polymerization process for low surface area pellets or chips, polymerization proceeds primarily by esterification with the diffusion of water. The slower diffusion of glycol by-product makes transesterification difficult. However, polymerization of high surface area powders can proceed by transesterification due to the fact that there is a reduced mean free path for glycol removal.
The low molecular weight polyester prepolymers utilized in solid state polymerizations are generally in the form of pellets, chips, or finely divided powder. Such pellets can vary greatly in size; however, as a general rule, the smaller the size of the pellets of polyester prepolymer the faster the solid state polymerization with proceed. Such polyester prepolymers are generally converted from the amorphous to the crystalline state prior to solid state polymerization in order to raise their sticking temperature. This is done in order to keep the pellets or chips of polyester prepolymer from sticking together as a solid mass in the solid state polymerization reactor.
In the solid state polymerization of a polyester prepolymer, the polymerization is carried out at an elevated temperature which is below the melting point of the polyester resin. Such polymerizations are normally conducted in the presence of a stream of inert gas or under a vacuum. Solid state polymerizations are normally conducted on a commercial basis in the presence of a stream of inert gas since it serves to remove volatile reaction products and helps to heat the polyester.
Heretofore, the form of the polyester prepolymer has essentially dictated the type of solid state polymerization process which could be employed in order to convert the low molecular weight polyester prepolymer into high molecular weight polyester resin. For example, it has generally been accepted practice to use polyester prepolymer in the form of pellets or chips in vacuum and static bed processes and finely ground powder in fluidized bed processes. The reason for this is that experience has shown that finely ground powders tend to agglomerate in vacuum processes, resulting in slower polymerization rates and a need to regrind the high molecular weight polyester resin produced. Experience has also shown that, in static bed processes finely ground powders will channel or fissure, resulting in uneven polymerization and prolonged polymerization rates. On the other hand, the use of pellets or chips in fluidized bed processes is not economically feasible in view of the velocity and volume of inert gas needed to suspend the pellets or chips and the size of the equipment required to do so.
Polyester prepolymers which are in the form of finely divided powders solid state polymerize at faster rates than do polyester prepolymers which are in the form of pellets or chips. However, polyester prepolymers which are in powder form are difficult to handle and generally must be polymerized in fluidized bed processes. Additionally, the high molecular weight polyester resins which are made utilizing prepolymers which are in powder form are also in the form of powders which are more difficult to process into articles of manufacture. For these reasons polyester prepolymers in powder form have not been widely utilized in commercial solid state polymerization techniques.
U.S. Pat. No. 4,876,326 discloses a process for solid state polymerizing a polyester prepolymer into a high molecular weight polyester resin comprising heating the polyester prepolymer to a temperature of from about 1.degree. C. to about 50.degree. C. below its sticking temperature, wherein the polyester prepolymer is in the form of porous pills, wherein said porous pills were made by agglomerating or compacting a powder of the polyester prepolymer having a particle size of less than about 841 microns. These porous pills are reported to have a substantial volume of interstice which penetrates the porous pills and open to the outside; with the porous pills having a density which is within the range of about 70% to about 99% of the density of solid polyester pellets which are comprised of the same polyester prepolymer. U. S. Pat. No. 4,957,945 discloses a process for preparing ultra-high molecular weight polyester resin which comprises: (1) dissolving a polyester prepolymer in a suitable organic solvent; (2) recovering the polyester prepolymer from the organic solvent to produce a porous, fibrous mass of the polyester prepolymer; and (3) solid state polymerizing the porous, fibrous mass at an elevated temperature to produce the ultra-high molecular weight polyester resin. U.S. Pat. No. 4,957,945 also reveals a process for preparing porous pills which can be solid state polymerized at a very rapid rate into high molecular weight polyester resin which comprises: (1) dissolving a polyester prepolymer in a suitable organic solvent; (2) recovering the polyester prepolymer from the organic solvent to produce a porous, fibrous mass of the polyester prepolymer; and (3) compacting the porous, fibrous mass into the porous pills.